The "turndown" ratio or capability of a burner is the ratio of its maximum firing rate to its minimum firing rate. Conventional ported premixed burners operating at fixed stoichiometry have a turndown capability of about 6:1. This ratio is generally governed by the flame speed of the particular fuel/air mixture, the flow velocity through the ports, the recirculation characteristics of the flow field immediately downstream from the ports, and the heat transfer characteristics of the system. If the relationship between these variables becomes out of balance, the result may be lifting or flashback. Upsets in the air/fuel ratio lead to increased emission of carbon monoxide, nitrous oxides, hydrocarbons, and other pollutants.
Burners are also employed in direct-fired make-up air furnace units. The burners are typically inserted into a make-up air duct or cabinet in a position upstream from an air blower. A portion of a make-up air stream is drawn into the burner(s) and enters a region where it is mixed with fuel injected through a manifold. The air/fuel ratio is chosen to minimize the emission of pollutants. The combustible mixture flows through a perforated plate which acts as a flame stabilizer. The hot combustion products then mix with bypass air, providing direct heat to the main air stream.
In order to increase the flexibility of premixed burners, to permit their operation over a wider range of temperatures and conditions, there is a need or desire for burners having higher turndown ratios that generate minimal quantities of pollutants.